I will compare and contrast feedforward inhibition in tow sets of thalamocortical projections into primary somatosensory cortex (SI) of the rat. The two projections stem from two distinct thalamic nuclei - the medial portion of the ventral posterior nucleus (VPM) and the medial portion of the posterior nucleus (Pom). These parallel thalamocortical pathways participate in sensory processing, motor processing, and in EEG rhythms generated during sleep. Feedforward inhibition probably plays a key role in these processes, but little is known about how it works. I will perform intracellular recording in a brain slice that contains both thalamic structures, SI, and the connections between the two, and study the synaptic and cellular properties of feedforward inhibitory circuitry in these two parallel projections. Very little is actually known about Pom, and from this information, I hope to acquire insight into its function. My specific aims are: 1) To describe the morphology and intrinsic membrane characteristics of inhibitory interneurons mediating feedforward inhibition in each thalamocortical pathway. 2) To describe and compare the functional characteristic of thalamocortical synapses on inhibitory interneurons mediating feedforward inhibition in each thalamocrotical pathway. 3) To characterize and compare disynaptic thalamocortical IPSPs onto layer III pyramidal cells from each thalamocortical pathway.